Efforts in the design of integrated circuits for radio frequency (RF) communication systems generally focus on improving performance, reducing cost or a combination thereof. One area of increasing interest relates to conversion of signals, such as from analog-to-digital or digital-to-analog. Both types of conversion have benefited from the development and use of delta-sigma modulation.
Delta-sigma modulation is a technique used to generate an estimate of a signal using a small number of quantization levels and a very high sampling rate. Limiting a signal to a finite number of levels introduces significant “quantization” noise into the system. Oversampling and the use of an integrator feedback-loop in delta-sigma modulation are effective in shifting quantization noise to out-of-band frequencies. The noise shifting properties enable efficient use of subsequent filtering stages to remove noise and produce a more precise representation of the input.
The filters within a delta-sigma modulator provide a noise transfer function (NTF) and a signal transfer function. The poles and zeroes of the NTF determine the shape and depth of low noise regions output by the modulator. The use of a delta-sigma modulator in a digital-to-analog conversion can produce an analog signal having a high dynamic range, but only for limited ranges of frequency. Outside of the narrow frequency bands having a high dynamic range, a delta-sigma modulated noise increases sharply. In effect, the delta-sigma modulator trades the majority of its spectral range for a few regions of high dynamic range operation. It is not generally possible to design the location and number of zeroes in the NTF to simultaneously control the width and depth of the high dynamic range (low noise) region(s) and the slope of the rise in noise at frequencies bordering the low noise region. Increased bandwidth can be achieved at the expense of higher order filters requiring additional circuitry and/or increased sample rates (which may require expensive materials such as Silicon Germanium or Indium Phosphide. An analog filter is required to remove the quantization noise from the out of band regions. It can be difficult and expensive to provide a filter that can respond to the rapid rise in noise at the boundaries of the high dynamic range regions without restricting the useful bandwidth of the signal.